The present invention is directed to processes and apparatus which quickly convert information from analog form (e.g., that which is contained on film, such as movie film or microfilm) to digital form. As the proliferation of electronic information continues, the capability to perform such a conversion at high speed would be beneficial in areas which require the digital version of large quantities of images presently contained in film media.
Conversion from film media to electronic form usually involves use of a charge couple device (CCD) image sensor. In particular, a linear CCD imager captures a one-dimensional image (i.e. it captures a row of pixels). A linear imager can provide high resolution and high speed scanning capability at a moderate cost. This type of device can be fabricated relatively inexpensively with high pixel counts, and high read-out speed. However, because a linear imager only captures one dimension of the target image, the underlying media must be externally scanned in an orthogonal direction to create a two-dimensional representation of the original film image.
All known prior methods of performing such a scanning operation suffer from either mechanical or physical drawbacks. In general, two approaches were used. The linear CCD would be held stationary while the film would be moved to scan the images, or the film would be held stationary while the linear CCD would be moved to scan the images. One example of the first method, depicted in FIG. 1, involves the use of a "nodding" mirror to do the beam sweeping. In this method, a mirror 12 moves slightly back and forth, resembling a nodding motion. This scans the underlying film 20 in two dimensions, with the linear CCD assembly 14 providing the scan in the y direction (i.e. perpendicular to the direction of film travel) and the nodding motion of the mirror 12 providing the scan in the x direction (i.e. parallel to the direction of film travel). However, the use of the mirror 12 introduces resolution limitations which necessitate the use of an asymmetrical field flattener 18 which provides correction for the arc of best focus 22. The need for the asymmetrical field flattener 18 results from the slight differences in the focal distance between the mirror 12 and the film 20, which are caused by the nodding motion of the mirror 12.
A second method according to the prior art involves holding the film 20 stationary and moving the linear CCD assembly 38 to perform the scan of the aerial image, as illustrated in FIG. 2. This method requires the linear CCD assembly 38 to accurately move through the image plane 40 to its final position 42. The close proximity required between the linear CCD assembly 38 and its signal processing and support circuits (not shown) means that the circuitry and associated cabling must be moved as well as the linear CCD assembly 38. This approach contains several drawbacks. First, the sheer bulk of the linear CCD assembly 38, its associated circuitry, and the mechanism necessary to move the assembly requires expensive mechanical translation components. In addition, these same factors limit the scanning speed of system operation. Furthermore, since the film 20 is held stationary in this method, only one image segment (i.e. frame of film) can be scanned at one time. Thus, once the current frame is scanned, the linear CCD assembly 38 must be moved from its final position 42 back to its starting point in preparation for scanning the next image. This limitation renders continuous film scanning impossible. One additional drawback to this method exists. In order to prevent focus errors in the aerial image 40, the film gate must maintain the film 20 in a precise location over the image area. This typically requires the use of optically suitable transparent `flats` 33 and 35 which constrain the film 20 to a known location. The flats 33 and 35 accumulate dirt transported by the film 20 or deposited by thermal air currents. The accumulation of this dirt can scratch the glass and, therefore, necessitates periodic cleaning, along with creating imperfections in reconstructed pictures.
A third method according to the prior art entails projecting a slit image of the film 20 onto a linear CCD assembly 70 as shown in FIG. 3. The film 20 moves longitudinally, presenting a succession of image parts through the scan slit. Synchronization between the film motion and the CCD readout clocking creates a series of signals from the successively read scan slit images which collectively represent the original image. Since the linear CCD assembly 70 remains stationary in this method, and since the film gate is very narrow, a continuous scan is possible. However, precise translational accuracy at the scan slit must be maintained in order to prevent distortion of the acquired image. Variations in film speed will vary the time duration of sequential lines which will result in objectionable `banding` in the image (i.e. noticeable variations in the image density). In order to avoid this effect, the prior art utilized a precision counter 72 to provide information on the film speed to a compensation circuit 74. The compensation circuit 74 would then attempt to counteract the adverse effects of film speed variations by properly indexing a buffer 76 of captured images, said indexed images then being fed on the data out line 78. Numerous factors, including but not limited to mechanical variations within the reel mechanism, the mass of the reels, the stretching of the film, and film tensioning variations within the reel render acquisition of an acceptable image impractical.